Metallic materials, more specifically iron and steel, are zinc-plated or cadmium-plated to protect them from corrosive environmental factors. The corrosion protection of zinc is due to the fact that it is even less precious than the base metal so that it first attracts all of the corrosive attack, thus acting as a sacrificial layer. The base metal of the zinc-plated component of concern remains intact as long as it remains completely covered with zinc, with the mechanical functionality being preserved longer than with parts that have not been zinc-plated. Thick zinc layers provide of course higher corrosion protection than thin layers—the corrosive removal of thicker layers taking longer.
Corrosive attack of the zinc layer may be heavily delayed by chromating so that corrosion of the base metal also is delayed further than with mere zinc-plating. Corrosion protection through the layer system zinc/chromating is much better than the one provided by a zinc layer that only has the same thickness. Further, chromating also defers optical erosion of a component part through environmental factors—the corrosion products of zinc, the so-called white rust, also affect the appearance of a component.
The advantages of chromating are so important that almost any galvanically zinc-plated surface is additionally chromated. Prior art knows of four chromating processes named by their colors and applied through processing (immersion, spraying, rolling) a zinc-coated surface with the corresponding aqueous chromating solution. Further, yellow and green chromatings, which are produced in an analogous manner, are known for aluminium. At any rate, the layers have different thicknesses and are substantially made from amorphous zinc/chromium oxide (or aluminium/chromium oxide) of a nonstoichiometric composition, a certain water content and incorporated foreign ions. The following chromating processes are known and classified in process groups according to DIN 50960, part 1:
1) Colorless and Blue Chromatings, Groups A and B:
The blue chromating layer is up to 80 nm thick, slightly blue in itself and has, depending on the layer thickness, a golden, reddish, bluish, greenish or yellow iridescent color produced by light refraction. Very thin chromate layers hardly having any color of their own are classified as colorless chromatings (group A). In both cases, the chromating solution may consist both of hexavalent and of trivalent chromates as well as of mixtures thereof, further of support electrolytes and of mineral acids. There are variants with fluoride and some without fluoride. The chromating solutions are used at room temperature. Corrosion protection of intact blue chromatings (group B) amounts to 10-40 h in the salt spray cabinet according to DIN 50021 SS before the first corrosion products appear. The minimum requirement for the process groups A and B according to DIN 50961, chapter 10, Table 3, is 8 h for workpieces placed in drums and 16 h for workpieces placed on racks.
2) Yellow Chromatings, Group C:
The yellow chromating layer is about 0.25-1 μm thick, dyed golden yellow and often highly purple-green iridescent. The chromating solution substantially consists of water-dissolved hexavalent chromates, support electrolytes and mineral acids. The yellow color is due to the significant fraction (80-220 mg/m2) of hexavalent chromium that is incorporated in addition to the trivalent chromium generated by reduction during the layer formation reaction. The chromating solutions are used at room temperature. Corrosion protection of intact yellow chromatings amounts to 100-200 h in the salt spray cabinet according to DIN 50021 SS before the first corrosion products appear. The minimum requirement for the Process Group C according to DIN 50961, chapter 10, Table 3, is of 72 h for workpieces placed in drums and 96 h for workpieces placed on racks.
3) Olive Chromating, Group D:
The typical olive chromating layer is of up to 1.5 μm thick, and is olive green to olive brown allover. The chromating solution substantially consists of water-dissolved hexavalent chromates, support electrolytes and mineral acids, more specifically of phosphates or phosphoric acid, and may also contain formates. Considerable amounts of chromium(VI) (300-400 mg/m2) are incorporated into the layer. The chromating solutions are used at room temperature. Corrosion protection of intact olive chromatings amounts to 200-400 h in the salt spray cabinet according to DIN 50021 SS before the first corrosion products appear. The minimum requirement for the Process Group D according to DIN 50961, chapter 10, Table 3, is of 72 h for workpieces placed in drums and 120 h for workpieces placed on racks.
4) Black Chromatings, Group F:
The black chromating layer basically is a yellow or olive chromating in which colloidal silver is incorporated as a pigment. The chromating solutions approximately have the same composition as yellow or olive chromatings and additionally contain silver ions. If the composition of the chromating solution is appropriate, iron, nickel or cobalt oxide incorporates into the chromate layer on zinc alloy layers such as Zn/Fe, Zn/Ni or Zn/Co as a black pigment so that silver is not necessary in this case. Considerable amounts of chromium(VI) are incorporated into the chromate layers in amounts of between 80 and 400 mg/m2, depending on whether the basis is a yellow or an olive chromating. The chromating solutions are used at room temperature. Corrosion protection of intact black chromatings on zinc amounts to 50-150 h in the salt spray cabinet according to DIN 50021 SS before the first corrosion products appear. The minimum requirement for the Process Group F according to DIN 50961, chapter 10, Table 3, is of 24 h for workpieces placed in drums and 48 h for workpieces placed on racks. Black chromatings on zinc alloys have considerably higher values than those mentioned.
5) Green Chromatings for Aluminium, Group E:
According to prior art, thick chromate layers with high corrosion protection >100 h in the salt spray cabinet according to DIN 50021 SS or ASTM 117-73 before the first corrosion products appear according to DIN 50961 (June 1987) chapter 10, more specifically chapter 10.2.1.2, may be manufactured without sealing and without any other particular post-treatment (DIN 50961, chapter 9), only by treatment with dissolved, markedly toxic chromium(VI) compounds. Accordingly, the chromate layers with the requirements mentioned placed on corrosion protection still contain these markedly toxic and carcinogenic chromium(VI) compounds that, in addition thereto, are not completely immobilized in the layer. Chromating with chromium(VI) compounds is problematic with regards to occupational safety and health. The use of zinc-plated chromatings made with chromium(VI) compounds, such as the widely used yellow chromatings on screws for example, constitutes a potential risk for the population and generally increases the risk of cancer.
Therefore, passivation methods obviating in part or in whole the use of chromium(VI) compounds are described in prior art.
U.S. Pat. No. 4,384,902 describes, with the examples 1, 2, 4 and 5 in particular, passivate layers meeting the requirements in the salt spray test. In all cases, the layer contains cerium having a yellowish coloration emphasized by the Ce(IV) ion. In the bath solution, the examples only contain Ce(III) and hydrogen peroxide as the oxidizing agent. The description discusses the fact that, under acid conditions, hydrogen peroxide does not act as an oxidizing agent for Ce(III), but that the surface pH increases so much during deposition for a sufficient amount of Ce(IV) to be generated. The yellowish color achieved with the bath composition described is indeed indicative of an oxidation, but only of an oxidation of Ce(III) to Ce(IV). Tetravalent cerium is a more efficient oxidizing agent than hexavalent chromium, this being the reason why Ce(IV) will produce Cr(VI) from Cr(III), which is to be avoided. Cr(VI) has a very strong yellow color and is known as a corrosion protection agent. Accordingly, the layer described in U.S. Pat. No. 4,384,902 is not free of hexavalent chromium.
U.S. Pat. No. 4,359,348 also describes passivate layers meeting the above mentioned requirements in the salt spray test. Again, in all the cases, the layer contains cerium having a yellowish coloration emphasized by the Ce(IV) ion. Therefore, this document does not go beyond U.S. Pat. No. 4,384,902.
Further, U.S patent application Ser.No. 2003/00234063 A1 discloses non-toxic corrosion-protection conversion coatings based on cobalt. These conversion coatings are described to be suitable for zinc substrates for example. The conversion coatings may, inter alia, contain Cr(III) ions and nicotinic acid.
Moreover U.S. Pat. No. 6,190,780 B1 discloses a surface treated metal material with corrosion-resistant coating layers. The metal material may be fused zinc-plated steel sheets. The conversion coating may contain Cr(OH)3 and nicotinic acid.
Further, GB-A-2 097 024 discloses the treatment of metal surfaces for improving corrosion protection on zinc and zinc alloy surfaces with an aqueous acidic solution containing an oxidizing agent and at least one metal, selected from the group consisting of iron, cobalt, nickel, molybdenum, manganese, aluminium, lanthanum, lanthanide mixtures or cerium ions or mixtures thereof and more specifically iron and cobalt ions. Further, GB-A-2 097 024 discloses the use of trivalent chromium ions and iron ions in combination with an additional metal, selected from the group consisting of the above mentioned ions or cerium ions, combinations of chromium(III) in combination with an oxidizing agent and cerium or lanthanum ions being mainly described though.
DE 196 15 664 A1 describes a method of producing chromium(VI)-free passivate layers having a greater layer thickness and increased corrosion protection. Organic chelate ligands, more specifically dicarboxylic acids, tricarbonic acids and hydroxycarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipinic acid, pimelic acid, suberic acid, azelaic acid, sebacinic acid, maleic acid, phthalic acid, terephthalic acid, ascorbic acid, malic acid, tartaric acid or citric acid, are thereby added to the reaction solution. These chelate ligands form complexes with chromium(III) having poor kinetic stability and quickly liberating said chromium which incorporates at high reaction speed into the growing ZnCrO layer. Metal ions such as bivalent cobalt ions in the form of soluble salts are added as an additional catalyst for increasing reaction speed and thickness growth of the chromate layer. The thus produced passivate layers do not contain any chromium(IV) and allow for corrosion protection of up to more than 100 h, which corresponds approximately to that of a conventional yellow chromating. The thus produced chromate layers have a greenish, purple-green iridescent color. An alternative method of passivation described in DE 41 35 524 A1, which relies on a chromium(III) oxalate complex, forms a blue passivate film.
DE 103 05 449 A1 describes a mixture of substances and a method of producing colored passivate layers, each of them relying on a combination of a reaction solution containing chromium(III) ions and of a quinoline dye. The disadvantage thereof is the poor stability of the quinoline dye both in the reaction solution and in the passivate layer. This is due, inter alia, to the lack of UV stability of such compounds.
The examples mentioned herein above show that chromium(III) passivations still only allow for restricted application. In addition to the often poor corrosion protection with blue chromatings and the risk of chromium(VI) residues, there also is the disadvantage that the possibilities of obtaining a coloring with chromium(III) passivations are limited. The colors obtained through chromium(III) passivation are substantially limited to blue and greenish layers of chromate, whilst yellow chromatings on the basis of chromium(III) will not allow to impart a uniform, strong yellow color, resulting instead in light, markedly iridescent coatings or to coatings tending to be bluish or greenish.
Repeated attempts have been made to produce yellow passivation layers only having a small chromium(VI) fraction or having no chromium(VI) at all. The intensive yellow color in conventional yellow chromatings is imparted by the very chromium(VI).
It is therefore an object of the present invention to provide a solution of producing passivate layers on a substrate, the solution containing chromium(III) but no chromium(VI).
It is another object of the present invention to provide a solution of producing passivate layers on a substrate, the layers being color intensive and durably stable yellow.
It is still another object of the present invention to provide a solution of producing passivate layers on a substrate, the layers being suitable to prevent corrosion of the substrate.
It is still another object of the present invention to provide a method of producing passivate layers on a substrate, the layers containing hardly any chromium(VI), being color intensive, durably stable yellow and being suitable to prevent corrosion of the substrate.